Method of cooling a gaseous mixture and installation therefor

ABSTRACT

A method of and apparatus for cooling and condensing a gaseous mixture by means of at least one frigorific cycle utilizing a cycle mixture which may include at least one constituent of said gaseous mixture, the cycle mixture being cooled and subjected to fractional condensation under high pressure; at least the first condensed fraction obtained during this fractional condensation is expanded to a low pressure lower than said high pressure; at least said expanded portion is vaporized and re-heated at said low pressure, in heat exchange relation with at least the cycle mixture in course of fractional condensation; at least the first heated portion is recompressed from said low pressure to said high pressure in at least one compression stage so as to reconstitute the cycle mixture, at least in part, under the high pressure, the first condensed fraction being obtained immediately after the last stage of compression; the said method further comprises the steps of expanding the portion of the first condensed fraction to the low pressure in at least one intermediate stage, consisting of expanding said portion from a pressure at most equal to the high pressure to a pressure intermediate between the said high and low pressures, a gaseous fraction from the said portion expanded to the intermediate pressure being separated out and re-compressed from the intermediate pressure to the high pressure in order to reconstitute a further portion of the cycle mixture under high pressure. The invention is especially applicable to the liquefaction of natural gas.

United States Patent [1 1 Darredeau Dec. 25, 1973 METHOD OF COOLING AGASEOUS MIXTURE AND INSTALLATION THEREFOR [75] Inventor: BernardDarredeau, Paris, France [73] Assignee: LAir Liquide, Societe AnonymePour LEtude Et LExploitation Des Procedes Georges Claude, Paris, France221 Filed: Dec. 20, 1971- [211 App]. No.: 209,810

[30] Foreign Application Priority Data Dec. 21, 1 970 France 7046084 52]US. Cl. 62/40, 62/9 [51] Int. Cl. F25j 1/00, F25j 3/06, F25j 5/00 [58]Field of Search 62/9, 11, 23, 40

[56] References Cited UNITED STATES PATENTS 3,593,535 7/l97l Gaume'r62/40 FOREIGN PATENTS OR APPLlCATlONS 665,385 6/1963 Canada 62/40Primary Examiner-Norman Yudkoff Assistant Examiner-Arthur F. PurcellA!t0rneylrvin S. Thompson et al.

[57] ABSTRACT A method of and apparatus for cooling and condensing agaseous mixture by means of at least one frigorific cycle utilizing acycle mixture which may include at least one constituent of said gaseousmixture, the cycle mixture being cooled and subjected to fractionalcondensation under high pressure; at least the first condensed fractionobtained during this fractional condensation is expanded to a lowpressure lower than said high pressure; at least said expanded portionis vaporized and re-heated at said low pressure, in heat exchangerelation with at least the cycle mixture in course of fractionalcondensation; at least the first heated portion is recompressed fromsaid low pressure to said high pressure in at least one compressionstage so as to re-constitute the cycle mixture, at least in part, underthe high pressure, the first condensed fraction being obtainedimmediately after the last stage of compression; the said method furthercomprises the steps of expanding the portion of the first condensedfraction to the low pressure in at least one intermediate stage,consisting of expanding said portion from a pressure at most equal tothe high pressure to a pressure intermediate between the said high andlow pressures, a gaseous fraction from the said portion expanded to theintermediate pressure being separated out and re-compressed from theintermediate pressure to the high pressure in order to re-constitute afurther portion of the cycle mixture under high pressure. The

' invention is especially applicable to the liquefaction of natural gas.

4 Claims, 6 Drawing Figures PM'ENIEDUEQS ms SHEEI 10F 6 PRIOR ARTPATENTED ntczs ms SHEET 2 OF 6 PATENTED DEC 25 4973 831T 3 OF 6PATENTEDUEMSIQH SHEET 5 BF 6 PRIOR ART PATENTEDnmzsms 3780.535

SHEET 6 a? s FIG. 6

INVENTlDN PRlOR ART The present invention has for its object a method ofcooling and condensing a gaseous mixture, together with an installationenabling the said method to be carried into effect. TI-le invention isespecially applicable to the liquefaction of natural gas.

At the International Refrigeration Congress of 1959 in Copenhagen, A.P.Kleemenko (Reports: pages 34 to 39) described a method of cooling andcondensing a gaseous mixture by means of a frigorific cycle utilizing acycle mixture which could comprise at least one constituent of thegaseous mixture treated.

In accordance with this method, at least the cycle mixture is cooled andsubjected to fractional condensation under high pressure, at least thefirst condensed fraction obtained during the said fractionalcondensation is expanded to a low pressure lower than the high pressure,at least the first expanded fraction is vaporized and heated under thelow pressure in heat exchange with the cycle mixture and the gaseousmixture in course of condensation, at least the first heated fraction isre-compressed from the low pressure to the high pressure so as tore-constitute, at least in part, the cycle mixture under the highpressure, the first condensed fraction being obtained immediately afterthe compression.

In addition, two distinct methods of operation of this cycle weredescribed. In a first case, the frigorific cycle is of the open type andthe gaseous mixture and the cycle mixture are combined and subjectedtogether to the fractional condensation. In a second case, thefrigorific cycle is of the closed type, and the cycle mixture and thegaseous mixture circulate in separate and distinct conduits, in whichthey are condensed independently.

Thie refrigeration cycle, known as auto-refrigerated cascade cycle, isnow well known. As compared with the Pictet cascade cycle, itnecessitates only a single compressor, and it is therefore distinguishedfrom this latter by smaller capital investments in equipment. Furthermore, certain improvements in this cycle have formed the subject ofFrench Patent No. 1,302,989 and it two Certificates of AdditionNos.80,294 and 86,485.

By way of example, a frigorific cycle of this kind, utilizing a cyclemixture having the following composition by volume:

Methane 35 percent Ethane 40 percent Propane 5 percent Butane 12 percentPentane 3 percent Nitrogen and other light gases 5 percent makes itpossible to liquefy and sub-cool a natural gas having the followingcomposition by volume:

Methane 88 percent Ethane 5 percent Propane 3 percent Butane 2 percentNitrogen and other light gases 2 percent While the previously describedmethod, improved in accordance with the above-mentioned patents, and thecorresponding installations give satisfaction, it must however beobserved that the degree of irreversibility of certain operationalphases of the method employed remains relatively large andcorrespondingly increases the total power consumed in condensing thegaseous mixture treated.

In this connection, the Applicant has found, in the case of liquefactionof natural gas, that the difference in temperature existing between thecycle mixture during the course of fractional condensation and the cyclemixture in course of heating remains large, especially in the firstexchanger or hot exchanger of the installation, in which thevaporization of the first condensed fraction is effected and essentiallyin the central zone.

of this latter.

In other words, during the first exchange of heat which permits thetemperature of the cycle mixture to be reduced from the ambienttemperature towards a temperature zone ofthe order of -30C., thedifference in temperature existing in certain zones between thecondensation curve of the cycle mixture and the vaporization curve ofthis latter corresponds to a considerable irreversibility of the saidheat exchange and correspondingly increases thetotal power consumed bythe installation. 4

The difference in temperature found depends especially on the ratio ofthe high pressure at which the fractional condensation of the cyclemixture is effected, and the low pressure under which the vaporizationof the condensed fractions of the said mixture is effected. Certainimperatives, imposed furthermore, prevent any modification of the ratioof the high and low pressures of the frigorific cycle in order to reducecorrespondingly the temperature difference found above.

Within the framework of an installation utilizing the so-calledauto-refrigerated cascade cycle, the present invention thus has theobject of reducing this difference of temperature existing between thecycle mixture in course of condensation and the said mixture in courseof vaporization, in the first exchanger or hot exchanger, in order toreduce the consumption of power necessary for the liquefaction of thegaseous mixture treated, and without having per contra any excessiveincrease in the exchange surface area of the said exchanger.

In order to achieve this result, a method according to the invention ischaracterized in that at least a portion of the first condensed fractionis expanded to the low pressure of the frigorific cycle in at least oneintermediate stage consisting of expanding the said portion from apressure at most equal to the high pressure of the frigorific cycle to apressure intermediate between the high pressure and the low pressure,separating a gaseous fraction from the said portion expanded to theintermediate pressure and recompressing the separated gaseous fractionfrom the intermediate pressure to the high pressure, so as tore-constitute another portion of the cycle mixture under the highpressure.

Advantageously, when at least the re-heated portion of the firstcondensed fraction is re-compressed to the high pressure in at least onestage of compression carried out from a pressure at least equal to thelow pressure to a mean pressure comprised between the low and highpressures, the said mean pressure is chosen as the intermediate pressureof the expansion stage. This makes it possible to combine, at the saidmean pressure, the gaseous fractions separated from the said portion ofthe first fraction and at least the said re-heated portion, and then tore-compress them together at the high pressure, in at least one otherstage of compression effected from the mean pressure to a pressure atmost equal to the high pressure.

Preferably, at least the re-heated portion of the first condensedfraction is re-compressed in two stages of compression. In this case,the said portion of the first condensed fraction is expanded to the lowpressure in a single intermediate stage, and the dividing pressurebetween the two compression stages is chosen as the intermediateexpansion pressure.

As compared with the known method previously described, the inventionmakes it possible in particular to enrich the first condensed fractionof the cycle mixture in heavy constituents, and therefore inconstituents having a high boiling point.

Correspondingly, the vaporization of the first condensed fraction iseffected in a first exchanger, or hot exchanger, at a temperature whichis everywhere higher than that previously obtained. At the level of thefirst exchange of heat, the difference of temperature between thevaporization curve and the condensation curve of the cycle mixture isthus correspondingly reduced. The thermodynamic efficiency of this firstexchange is therefore improved and in consequence, the correspondingpower consumption of the installation is reduced.

As compared with a conventional auto-refrigerated cascade cycleinstallation, the installation corresponding to the method according tothe invention necessitates only a small additional investment. On theone hand, it is in fact found that the differences in temperatureinitially encountered in the hot exchanger being large, their relativereduction, obtained according to the invention, remains small. Theresult is therefore that the exchange surface necessary for the firstexchanger is only very slightly increased. Furthermore, in certaincases, as the invention makes it possible to harmonize the vaporizationand condensation curves of the cycle mixture, there results a betterharmonization of the differences in temperature along the hot exchanger,and so the exchange surface area may remain unchanged. On the otherhand, it must be observed that, as the gaseous fraction separated fromthe first condensed fraction at the intermediate pressure is not large,the corresponding intermediate separator remains of modest dimensions.

In addition, in an installation which carries into effect a methodaccording to the invention, the flow-rate treated in the lastcompression stage is always larger as that of the first stage. Thisalways leads therefore, according to the invention, to a betteradaptation of the compression unit and this advantage is especiallyappreciable in the case of a single compressor of the axial type.

Other objects and advantages of the present invention will becomeapparent from examination of the detailed description which followsbelow, with reference to the accompanying drawings, in which the samereference numbers have been given to the same parts.

In the drawings:

FIG. 1 represents an installation for carrying into effect the so-calledauto-refrigerated cascade cycle;

FIGS. 2, 3 and 4 show three installations for following this same cycle,as improved according to the invention;

FIG. 5 shows heat exchange diagrams illustrating the theoreticalconsiderations previously referred to. In these diagrams, the coolingand heating curves relating to the first exchanger or hot exchanger ofan autorefrigerated cascade frigorific installation have been drawn. Tothis end, the quantities of heat (0) in kilocalories have been plottedin ordinates and the temperatures in degrees Celsius are plotted inabscissae. The curves in full lines correspond to the exchange diagramof a hot exchanger of an installation according to FIG. 1, and thereforeof a conventional auto-refrigerated cascade installation. The curves inbroken lines correspond to the exchange diagram of a hot exchanger in aninstallation improved according to the invention, as shown in FIG. 2,under conditions of delivery output from the compressor, of flow-rate ofthe gaseous mixture treated (natural gas) and pressures identical withthose taken into consideration for FIG. 1;

FIG. 6 represents the total exchange surface S (not including theexchange surface of the final condenser arranged after the compressor),expressed in relative values (that is to say to liquefy l Nm of naturalgas), necessary in the case of FIGS. 1, 2 and 4, as a function of thepower P to be supplied to the cycle mixture.

A conventional installation of the auto-refrigerated cascade type,permitting the cooling and condensation of a gaseous mixture such asnatural gas, comprises a frigorific unit such as that shown in FIG. I,intended for the circulation of a cycle mixture comprising, if sodesired, at least one constituent of the gaseous mixture treated. In thecase of liquefaction of natural gas, the cycle mixture comprises acertain number of hydrocarbons of the gas to be liquefied (methane,ethane, propane, etc.) and, when so desired, nitrogen, depending on thecooling desired.

The refrigeration unit shown in FIG. 1 comprises a compressor 2, inwhich the suction and the delivery work under pressures respectivelytermed hereinafter as low pressure and high pressure". The compressor 2comprises a first compression stage 2 sucking-in at low pressure anddelivering at a means pressure comprised between the high and lowpressures, a second and last stage 2" sucking-in at the mean pressureand delivering at the high pressure.

A final condenser 3", the inlet of which communicates with the deliveryof the'compressor 2 is associated with this latter. It comprisescirculating means for a refrigerant external to the frigorific unit,such as water. A first exchanger 10 or hot exchanger, a second exchanger20, a third exchanger 30, a first separator 3, a second separator 13, afirst expansion valve 4', a second valve 14, a third valve 15, permitthe continuation of the fractional condensation of the cycle mixtureutilized, commenced in the condenser 3".

The inlet of the first separator 3 communicates with the outlet of thecondenser 3". Each exchanger 10 or 20 comprises a first passage means 51communicating at one extremity with the gaseous outlet of a separator 3or 13, and at the other extremity with the inlet of the second separator13 (cf exchanger 10) or the third expansion valve 15' (cf exchanger 20);a second passage means 52, constituted by the interior of each exchanger10 or 20, in heat exchange relation with the first passage means 51,communicating with the downstream side of an expansion valve 4 or 14'and with the suction side of the compressor 2 through the conduit 6 orthrough the conduits l6 and 26; a third passage means 53 for the gaseousmixture to be cooled and liquefied, in thermal exchange relation withthe second passage means 52; a fourth passage means 54 in heat exchangerelation with the second passage means 52, of which one extremitycommunicates with the liquid outlet of a separator 3 or 13, while theother extremity communicates with the upstream side of an expansionvalve 4' or 14'.

Each expansion means associated with each separator 3 or 13, comprisingan expansion valve 4 or 14', thus communicates at its upstream portionwith the liquid outlet of a separator 3 or 13, through the intermediaryof a fourth passage means 54 of an exchanger or 20, and at itsdownstream portion with a second passage means 52 of an exchanger 10 or20.

The exchanger 30 differs from the other exchangers l0 and 20 in that itis not provided with a fourth passage means 54, and in that its passagemeans 51, previously specified, communicates directly at one extremitywith the third expansion valve without the intermediary of a separatorsimilar to the separators 3 and 13, and at the other extremity with thefirst passage means 51 of the second exchanger 20.

In operation, following the frigorific cycle described in FIG. 1, thecycle mixture previously described, issuing from the compressor 2 at thehigh pressure of 40 bars, is cooled and subjected to fractionalcondensation. For that purpose, it is first partly condensed by passinginto the condenser 3". Then, when it reaches the first separator 3, thefirst condensed fraction obtained in the condenser 3" is separated fromthe remainder of the cycle mixture.

The first condensed fraction is evacuated from the separator 3 by theconduit 4, sub-cooled by passing into the fourth passage means 54 of theexchanger 10, expanded to the low pressure of 2.5 bars in an expansionmeans comprising the first expansion valve 4', led through the conduit4" into the exchanger 10, vaporized and heated by passage into thesecond passage means 52 of the said exchanger, by heat-exchange incounter-flow with at least the first condensed fraction in course ofsub-cooling, and finally evacuated from the exchanger 10 through theconduit 6.

The remainder of the cycle mixture is evacuated from the first separator3 and its fractional condensation is continued by passing into the firstpassage means 51 of the exchanger 10, by heat exchange in counterflowwith the first condensed fraction in course of vaporization and heatinginthe second passage means 52.

The cycle mixture is then evacuated from the exchanger 10 through theconduit 5, and led to the second separator 13, in which a secondcondensed fraction is separated from the cycle mixture.

With regard to the gaseous mixture (natural gas) to be cooled andcondensed, this is introduced through the conduit 1 into the thirdpassage means 53 of the exchanger 10. It is then cooled by exchange ofheat in counter-flow with the first fraction condensed and expanded tothe low pressure, in course of vaporization, circulating in the secondpassage means 52 of the exchanger 10.

The second condensed fraction is evacuated from the separator 13 throughthe conduit 14, sub-cooled by passage into the fourth passage means 54of the exchanger 20, expanded to the low pressure in an expansion meanscomprising the second expansion valve 14', led by the conduit 14" intothe exchanger 20, vaporized and heated by passing into the secondpassage means 52 of the said exchanger, by heat exchange in counter-flowwith at least the second condensed fraction in course of sub-cooling andfinally evacuated from the exchanger 20 by the conduit 16.

The cycle mixture, remaining in the gaseous state, is evacuated from thesecond separator 13 by the conduit 15, and its fractional condensationis continued by passing into the first passage means 51 of the secondexchanger 20, by heat exchange in counter-flow with the second fractioncondensed during the course of vaporization and heating in the secondpassage means 52. The cycle mixture is then evacuated from the exchanger20 towards the first passage means 51 of the third exchanger 30. Withregard to the gaseous mixture (natural gas), this continues itscondensation at a temperature level lower than that of the firstexchanger 10, in the third passage means 53 of the second exchanger 20,by' exchange of heat in counter-flow with the second fraction condensedand expanded to the low pressure, during the course of vaporization inthe second passage means 52 of the exchanger 20.

The cycle mixture completes its condensation, and becomes sub-cooled bypassing into the first passage means 51 of the third exchanger 30. Thethird condensed fraction thus obtained, sub-cooled, is expanded to thelow pressure in the third expansion valve 15", is vaporized and heatedin the second passage means 52 of the third exchanger 30 by exchange ofheat in counter-flow with at least the remainder of the cycle mixture atthe end of the fractional condensation, circulating in the first passagemeans 51, and is evacuated from the exchanger 30 by the conduit 26.

The gaseous mixture (natural gas) completes its condensation' at atemperature level lower than that of the second exchanger 20 by passinginto the third passage means 53 of the exchanger 30, by exchange of heatin counter-flow with the last condensed fraction of the cycle mixture incourse of vaporization. It may be subcooled, if so required, in thethird exchanger 30. The condensed gaseous mixture, sub-cooled if sodesired, is evacuated from the frigorific unit and expanded to itsproduction pressure in the expansion valve 56.

As regards the three condensed fractions of the cycle mixture, vaporizedrespectively in the exchangers 10, 20 and 30, they are combined by meansof the conduits 6, 16 and 26 and are sent back to the suction of thecompressor 2, after passing into a safety separator 55. They are thenre-compressed from the low pressure (2.5 bars) to the high pressure (40bars) of the cycle in order to re-constitute the cycle mixture underhigh pressure. Their compression is carried out in a first stepcompleted in the first stage 2' from the low pressure to a meanspressure, and in a second and last step carried out in the second stage2", from the means pressure to the high pressure.

FIG. 2 represents a frigorific unit similar to that previouslydescribed, but modified according to the invention. As has beenpreviously explained, this modification concerns solely the expansionmeans associated with a separator of the portion of the frigorific unitin which the fractional condensation of the cycle mixture is effected.

In accordance with FIG. 2, the first expansion means associated with thefirst separator 3 comprises, in addition to the first expansion valve4', a single intermediate stage. This latter comprises an intermediateexpansion valve 104, working between the high pressure of the frigorificcycle and the mean cut-out pressure of the compressor 2, the upstreamportion of which communicates through the conduit 56 with the liquidoutlet from the first separator 3; an intermediate separator 103, theinlet of which communicates with the downstream side of the saidintermediate expansion valve 104, the gaseous outlet of whichcommunicates with the delivery of the first compression stage 2' of thecompressor 2 through the conduit 105, while the liquid outletcommunicates through the conduit 114 with the upstream side of the firstexpansion valve 4.

The operation of the installation described with reference to FIG. 2differs from that described in the installation shown in FIG. 1, only bythe method of expansion to the low pressure of the first fractioncondensed and collected in the separator 3. According to FIG. 2, thefirst condensed fraction extracted from the separator 3 through theconduit 56 is expanded to the low pressure with a single intermediatestep.

This step consists of expanding the first condensed fraction coming fromthe conduit 56 in the intermediate valve 104, to an intermediatepressure equal to the means delivery pressure of the first compressionstep 2'.

After this, a gaseous fraction is separated from fraction expanded tothe means pressure, in the separator 103. This fraction is evacuatedthrough the conduit 105, reunited at the mean pressure, at the deliveryof the first compression stage 2, with the heated portions of the cyclemixture, and recompressed with these latter to the high pressure, in thesecond compression stage 2 effected from the mean pressure to the saidhigh pressure in order to re-constitute another portion of the cyclemixture under high pressure.

In addition to the advantages previously indicated which assist inreducing the power expenditure in condensing the gaseous mixturetreated, the embodiment shown in FIG. 2 further contributes to animprovement of the economy of the frigorific cycle, for the followingreasons: on the one hand, the total flow-rate of the cycle mixture atthe delivery of the compressor 2 remains practically unchanged; thematerial balance is practically the same, except as regards the gaseousfraction obtained at the intermediate pressure in the separator 103, andwhich is sent at a lower temperature into the second stage 2" ofcompression.

On the other hand, the compression is relieved in the first stage 2 ofthe compressor 2, of all the gaseous fraction obtained at theintermediate pressure. If, for example, the rate of compression is thesame in both stages of the compressor 2, this gaseous fraction mayrepresent to 12 percent of the cycle mixture. In this case, the gain inpower is from 5 to 6 percent.

The frigorific unit shown in FIG. 3 is differentiated from the unitshown in FIG. 2 by the fact that the intermediate expansion stagedescribed in FIG. 2 further comprises, according to FIG. 3, anintermediate heat exchanger 200.

This exchanger 200 comprises a first passage means 57 constituted by theinterior of the said exchanger, communicating with the downstream sideof the intermediate expansion valve 104 and with the input of theintermediate separator 103; a second passage means 58 communicating atone extremity with the liquid outlet of the first separator 3 and at theother extremity with the upstream side of the intermediate expansionvalve 104, in heat-exchange relation with the first passage means 57; athird passage means 59, in heat-exchange relation with the first passagemeans 57, communicating at one extremity with the liquid outlet of theintermediate separator 103, and at the other extremity with the upstreamside of the first expansion valve 4', through the intermediary of thefourth passage means 54 of the first exchanger 10; a further passagemeans 60, in heat-exchange relation with the first passage means 57 forall flows in course of cooling.

The operation of the frigorific unit according to FIG. 3 isdistinguished from that of the unit previously described, solely by theexchange of heat which takes place in the exchanger 200. In this latter,there is vaporized, at least partially, in the first passage means 57,the first condensed fraction expanded to the intermediate pressure inthe valve 104, and passing into the exchanger through the conduit 204".

The necessary heat of vaporization is obtained by exchange of heat,firstly with the first condensed fraction in course of sub-coolingbefore its expansion 104 to the intermediate pressure, circulating inthe second passage means 58 of the exchanger 200 and coming from thefirst separator 3; secondly, with the first condensed fraction,separated from the gaseous fraction derived from the intermediateseparator 103 through the conduit 114, and circulating in the thirdpassage means 59 of the exchanger so as to be sub-cooled before itsexpansion to a lower pressure, equal to the low pressure, in the firstexpansion valve 4; thirdly, with another flow in course of cooling,passing into the exchanger 200 through the conduit 201, and circulatingin the other passage means 60.

This other flow may be the cycle mixture derived from the gaseous outletof the first separator 3, the gaseous mixture to be cooled and condensed(natural gas for example), or any other fluid at a temperature in theneighbourhood of ambient temperature, which it is necessary to cool.

It is found that the frigorific unit according to FIG. 3, by comparisonwith that of FIG. 1, makes it possible to obtain a still greater gain inpower with respect to that of FIG. 2. In fact, the first condensedfraction of the cycle mixture being at least partly vaporized in theintermediate heat exchanger, on the one hand the percentage of thegaseous fraction separated in the separator 103 is considerablyincreased, and on the other hand the first condensed fraction is stillfurther enriched in heavy constituents.

In additiion, all the cold generated in the heatexchanger 200 is onlyhalf as expensive in energy, since the compression of the cycle mixturecan be reduced by half. The exchange surface necessary is of courseincreased as the vaporization of the first condensed fraction becomesgreater. This imposes a limit on the gain of power which can beobtained. It can however be of the order of 10 percent.

The frigorific unit shown in FIG. 4 differs from that shown in FIG. 3only by the fact that the intermediate exchanger 200 also comprises afourth passage means 61 communicating at one extremity with the gaseousoutlet of the first separator 3, and at the other extremity with theinlet of the second separator 13, through the intermediary of the firstpassage means 51 of the first exchanger 10, and a fifth passage means 62permitting the cooling of the gaseous mixture treated to be started,communicating at one extremity with the third passage means 53 of thefirst exchanger 10.

In consequence, according to FIG. 4, the heat required for thevaporization of the first condensed frac- 9 tion in the exchanger 200 isalso supplied by exchange of heat in counter-flow with the cycle mixturein course of fractional condensation, coming from the first separator 3and circulating in the fourth passage means 61, and by exchange of heatin counter-flow with the gaseous mixture (natural gas) in course ofcooling, circulating in the fifth passage means 62 of the exchanger 200and flowing towards the exchangers 10, 20 and 30.

Analysis of the exchange diagrams shown in FIG. makes it possible toillustrate the theoretical considerations postulated above. In thisfigure, the cooling curves (arrows pointing downwards) represent the sumofthe quantities of heat exchanged in the gaseous mixture 1 (naturalgas) in course of cooling and condensation, in the cycle mixture 5 incourse of cooling and fractional condensation, and in the firstcondensed fraction coming from the first separator 3 in course ofsub-cooling.

As regards the heating curves (arrows pointing upwards), they representthe quantity of heat exchanged by the cycle mixturein course of heating,coming-in through the conduits 16 and 4", comprising the first condensedfraction in course of vaporization and heating at the low pressure.

Referring now to the curves in full lines (FIG. 1) that is to say in thecase of a conventional autorefrigerated cascade" unit, it is found thatthe cooling curve is a substantially linear function of the temperature,and that the heating curve comprises an angular point, corresponding toan abrupt and considerable change in slope in the central zone of thefirst exchanger 10. This results ina substantial difference intemperature, essentially in this zone of the exchanger, which affectsthe thernio-dynamic efficiency of the frigorific cycle.

Referring now to the curves in broken lines corresponding to FIG. 2,that is to say in the case of an improved frigorific'unit according tothe invention, it is found on the one hand that the heating curveapproaches the cooling curve, and on the other hand that the heatingcurve is much'flatter than in the previous case. The difference intemperature has therefore been reduced over the whole length of thefirst exchanger or hot exchanger, and'this essentially in the centralzone of this exchanger. The reversibility of the first exchange of heatis thus increased, and this contributes to reducing the power consumedin liquefying the gaseous mixture treated.

The curves of FIG. 6 bring out clearly the gain obtained according tothe invention, for an equal exchange surface or an equal expenditure ofpower.

The curves VAl, VA2 and VA4 related respectively to the case of FIGS. 1,2 and 4. A comparison of these curves will show:

- that, as compared with the case of FIG. 1, that of FIG. 2 results ingains of power of about 5 percent with equal exchange surface, and 6 to10 percent in exchange surface for equal expanditure of power;

- that, the choice between the case of FIG. 2 and that of FIG. 4 must bemade for each case as a function of economic criteria, the case of FIGS.3 and 4 being essentially employed when the power is expensive.

It will of course be understood that the present invention is not in anyway limited to the forms of embodiment described and shown. It iscapable of receiving sion unit comprising a number of compressors each'forming one stage of compression.

Secondly, each of the condensed fractions of the cycle mixture iscapable, in the same manner as for the first condensed fraction, ofbeing expanded to the low pressure of the frigorific cycle in at leastone intermediate stage, so as to obtain the same advantages of theinvention at the level of the various exchangers 20 and 30 of thefrigorific unit.

Thirdly, the invention is applicable to an autorefrigerated cascadecycle, whether this is of the open or closed type.

Fourthly, the invention is not limited to a cycle in which the firstrefrigeration, in the condenser located immediately at the outlet of thecompressor, is effected with a refrigerant such as water. Depending onthe case, this initial refrigeration may be carried out with afrigorific cycle independent of the auto-refrigerated cascade cycle,utilizing for example, propane as the refrigerant fluid.

What I claim is:

1. In an auto-refrigerated cascade method of cool-ing and condensing agaseous mixture (1) by means of a frigorific cycle utilizing a cyclemixture said frigorific cycle comprising:

a. partially condensing (3') said cycle mixture under a high pressure byheat exchange with an external refrigerant,

b. separating (3) a first condensed fraction from said partiallycondensed cycle mixture,

c. fractionately condensing (13) under said high pressure the remainder(5) of said cycle mixture separated from said first condensed fraction,and obtaining thereby a second condensed fraction d. expanding (4') atleast a portion of said first condensed fraction and at least a portionof said second condensed fraction (14') to a low pressure lower thansaid highpressure,

e. vaporizing and reheating (52) at least said expanded portions undersaid low pressure, in heat exchange (51) with at least said remainingcycle mixture in the course of fractional condensation,

f. recompressing (2) at least said reheated portions (6) from said lowpressure to said high pressure in at least one compression stage (2"),so as to reconstitute, at least in partysaid cycle mixture under saidhigh pressure, and effecting the partial condensation (3) of step a)immediately after the last stage (2") of compression;

the improvement comprising g. serially expandingduring step d) saidportion of said first condensed fraction (56) in at least oneintermediate stage from said high pressure to a pressure intermediatesaid high pressure and said low pressure,

h. separating (103) out a gaseous fraction (105) from said portion (114)expanded to said intermediate pressure,

i. recompressing (2") said separated gaseous fraction (105) from saidintermediate pressure to said high pressure, in order to reconstitute afurther portion of said cycle mixture under high pressure, and

j. expanding (4') said portion (114) separated during step h) from saidgaseous fraction (105), from said intermediate pressure to a pressure atleast equal to said low pressure.

2. A method as claimed in claim 1, in which said portion (264") expandedto said intermediate pressure, resulting from step (g), is partlyvaporized (57), prior to the separation step (h), by heat exchange withsaid portion (1 14) separated during step (h) from said gaseous fraction(105), in the course of subcooling (59) prior to its expansion (in valve4') according to step (j), and with at least one other flow of fluid(201) in the course of cooling.

3. An auto-refrigerated cascade installation for cooling and condensinga gaseous mixture (1), comprising a frigorific unit for the circulationof a cycle mixture, said frigorific unit comprising:

- a compressor (2) having its suction and delivery operatingrespectively under a low pressure and a high pressure, comprising afinal compression stage (2), the suction and delivery of which operaterespectively at a pressure intermediate said high and low pressures andat said high pressure;

- a condenser (3') for cooling and partial condensation under said highpressure of at least said cycle mixture, the input of said condenser(3') communicating with the delivery of said compressor (2), andcomprising circulation means for an external refrigerant to saidfrigorific unit;

- a first separator (3) for the separation of a first condensed fraction(56) from said cycle mixture par tially condensed in said condenser, theinlet of said separator (3) communicating with the outlet of saidcondenser (3');

- first expansion means (104, 103, 4), comprising one first expansionvalve (4') for the expansion to said low pressure of at least a portionof said first condensed fraction (56), and the upstream side of which isin communication with the liquid outlet of said first separator (3);

- at least one heat exchanger for the fractional condensation under saidhigh pressure of at least the remainder (5) of said cycle mixture,separated from said first condensed fraction (56), and comprising: afirst passage means (51) for the cycle mixture (5) in the course offractional condensation, communicating at one extremity with the gaseousoutlet of said first separator (3), and at the other extremity with theinlet (5') of a second separator (13); a second passage means (52) forthe portion of said first condensed fraction (4"), expanded to said lowpressure and in course of vaporization and heating, in heat exchangerelation with said first passage means (51), communicating with thedownstream side of said first expansion valve (4) and with the suctionof said compressor (2); and a third passage means (53) for said gaseousmixture (1) in the course of cooling and condensation, in heat exchangerelation with said second passage means (52);

- said first expansion means (104, 103, 4') being in fluid series andcomprising an intermediate stage comprising an intermediate expansionvalve (104) for the expansion of said portion (56) of the firstcondensed fraction from said high pressure to a pressure intermediatesaid high and low pressures; and

- an intermediate separator (103) for the separation of a gaseousfraction from said portion (56) expanded to said intermediate pressure,the inlet of which communicates with the downstream side of saidintermediate expansion valve (104), the gaseous outlet (105) of whichcommunicates with the suction of the last compressionstage (2") of saidcompressor (2), so as to combine under said intermediate pressure, saidvaporized and heated portion and said gaseous fraction (105) and torecompress them to said high pressure, and the liquid outlet (114) ofwhich communicates with the upstream side of said first expansion valve(4').

4. An installation as claimed in claim 3, in which said intermediatestage comprises an intermediate heat exchanger (200) for the partialvaporization of the portion (204") of said first fraction expanded (104)to the intermediate pressure, and comprising: a first passage means(57), for said expanded portion (204) in the course of vaporization,communicating with the downstream side of said intermediate expansionvalve (104) and with the inlet of said intermediate separator (103), asecond passage means (59) for the portion (114) expanded to saidintermediate pressure and separated from the gaseous fraction (105), inthe course of subcooling, in heat exchange relation with said firstpassage means (57), communicating at one extremity with the liquidoutlet (114) of saidintermediate separator (103), and at the otherextremity with the upstream side of said first expansion valve (4'); andat least a further passage means (60) for another flow (201) in thecourse of cooling, in heat exchange relation with said first passagemeans (57).

2. A method as claimed in claim 1, in which said portion (264'''')expanded to said intermediate pressure, resulting from step (g), ispartly vaporized (57), prior to the separation step (h), by heatexchange with said portion (114) separated during step (h) from saidgaseous fraction (105), in the course of subcooling (59) prior to itsexpansion (in valve 4'') according to step (j), and with at least oneother flow of fluid (201) in the course of cooling.
 3. Anauto-refrigerated cascade installation for cooling and condensing agaseous mixture (1), comprising a frigorific unit for the circulation ofa cycle mixture, said frigorific unit comprising: - a compressor (2)having its suction and delivery operating respectively under a lowpressure and a high pressure, comprising a final compression stage(2''), the suction and delivery of which operate respectively at apressure intermediate said high and low pressures and at said highpressure; - a condenser (3'') for cooling and partial condensation undersaid high pressure of at least said cycle mixture, the input of saidcondenser (3'') communicating with the delivery of said compressor (2),and comprising circulation means for an external refrigerant to saidfrigorific unit; - a first separator (3) for the separation of a firstcondensed fraction (56) from said cycle mixture partially condensed insaid condenser, the inlet of said separator (3) communicating with theoutlet of said condenser (3''); - first expansion means (104, 103, 4''),comprising one first expansion valve (4'') for the expansion to said lowpressure of at least a portion of said first condensed fraction (56),and the upstream side of which is in communication with the liquidoutlet of said first separator (3); - at least one heat exchanger (10)for the fractional condensation under said high pressure of at least theremainder (5) of said cycle mixture, separated from said first condensedfraction (56), and comprising: a first passage means (51) for the cyclemixture (5) in the course of fractional condensation, communicating atone extremity with the gaseous outlet of said first separator (3), andat the other extremity with the inlet (5'') of a second separator (13);a second passage means (52) for the portion of said first condensedfraction (4''''), expanded to said low pressure and in course ofvaporization and heating, in heat exchange relation with said firstpassage means (51), communicating with the downstream side of said firstexpansion valve (4'') and with the suction of said compressor (2); and athird passage means (53) for said gaseous mixture (1) in the course ofcooling and condensation, in heat exchange relation with said secondpassage means (52); - said first expansion means (104, 103, 4'') beingin fluid series and comprising an intermediate stage comprising anintermediate expansion valve (104) for the expansion of said portion(56) of the first condensed fraction from said high pressure to apressure intermediate said high and low pressures; and - an intermediateseparator (103) for the separation of a gaseous fraction (105) from saidportion (56) expanded to said intermediate pressure, the inlet of whichcommunicates with the downstream side of said intermediate expansionvalve (104), the gaseous outlet (105) of which communicates with thesuction of the last compression Stage (2'''') of said compressor (2), soas to combine under said intermediate pressure, said vaporized andheated portion and said gaseous fraction (105) and to recompress them tosaid high pressure, and the liquid outlet (114) of which communicateswith the upstream side of said first expansion valve (4'').
 4. Aninstallation as claimed in claim 3, in which said intermediate stagecomprises an intermediate heat exchanger (200) for the partialvaporization of the portion (204'''') of said first fraction expanded(104) to the intermediate pressure, and comprising: a first passagemeans (57), for said expanded portion (204'''') in the course ofvaporization, communicating with the downstream side of saidintermediate expansion valve (104) and with the inlet of saidintermediate separator (103), a second passage means (59) for theportion (114) expanded to said intermediate pressure and separated fromthe gaseous fraction (105), in the course of subcooling, in heatexchange relation with said first passage means (57), communicating atone extremity with the liquid outlet (114) of said intermediateseparator (103), and at the other extremity with the upstream side ofsaid first expansion valve (4''); and at least a further passage means(60) for another flow (201) in the course of cooling, in heat exchangerelation with said first passage means (57).